Recently I upgrade one website from 1.8.1.0 to 1.9.4.0. Everything working fine into the website accept one issue.
I face strange issue, The customer who make the payment using "Paypal Express Checkout" are not able to login into the website.
After successfully make the payment for the order, customer register sucessfully.
Now when they try to login with credential, they are not able to login into the website.
There is not any error on page, log file or etc.